CAPT OF AN MLG FAILED TO MAINTAIN ATC ASSIGNED SPD RESTR AFTER CRUISE LEVEL OFF.

Narrative

ON FLT FROM PHX TO LAX; ZAB ASSIGNED 250 KTS ON OUR CLBOUT. THE CAPT WAS FLYING AND INPUT THE AIRSPD IN THE FMC. AS I WAS ABOUT TO CONFIRM THE FMC ENTRY; ATC GAVE A FREQ CHANGE AND THE FLT ATTENDANT ENTERED THE COKPIT WITH COFFEE FOR THE CAPT. AFTER I CHKED IN WITH CTR AND TOOK THE COFFEE; I BEGAN DOING THE PAPERWORK AND GETTING THE APCH CHARTS OUT FOR LAX MEANWHILE; WE LEVELED AT FL280. I WAS STILL DOING OTHER DUTIES AND DID NOTICE THE AIRSPD INCREASE TO OVER 300 KTS. ZAB CALLED AND ASKED IF WE WERE DOING 250 KTS. I SAID NO -- WE ARE SLOWING NOW. ATC THEN DIRECTED AN IMMEDIATE DSCNT TO FL260 AND RPT OUT OF FL270. THE CAPT WAS INPUTTING IT IN THE FMC AND I SAID YOU NEED TO DSND NOW! SHE SAID WE ARE DSNDING. AFTER LEVEL OFF AT FL260; I NOTED TFC AT FL280 5 NM AT OUR 12 O'CLOCK ON TCASII. NOTHING MORE WAS MENTIONED BY ATC. I FEEL THAT WHEN AIRSPD CLRNC IS RECEIVED; IT SHOULD BE INPUT IN ALL PHASES OF FLT IN THE FMC -- NOT JUST THE PRESENT PHASE; ATC WAS ON TOP OF THINGS. I ONLY WISH WE HAD BEEN.